1. Field of the Invention
The present invention relates to cutting tools, more particularly to cutting tool inserts for machining hardened steel.
2. Prior Art
It is well-known to form superhard cutting tools from a carbide tool insert having a pocket or slot defined therein. A superhard cutting element, sometimes referred to as a blank, is fixed into the pocket, typically by brazing. It is also known to coat cutting tools with a wear-resistant layer to enhance the life of the cutting tool. The material of such cutting tool substrates have included tool steels, cemented carbides, cermets, ceramics, ceramic composites, polycrystalline diamond and polycrystalline cubic boron nitride. The substrates are typically coated with a wear-resistant compound such as TiN, TiCN, TiOCN, TiC, Al2O3 or a combination thereof.
The life of the cutting tool is often related to the thickness of the wear-resistant coating. Thicker coatings wear through to the substrate slower than thinner coatings. One problem with thicker PVD coatings (i.e., greater than 5 micrometers) is that high stress distributions develop in the coating which may cause the coating to
One solution to the problem of coating delamination was proposed in commonly owned U.S. Pat. No. 5,722,803, which is incorporated by reference herein. The ""803 patent describes PVD TiAlN coatings with thicknesses of greater than 5 micrometers deposited on cemented carbide substrates, particularly cobalt cemented tungsten carbide. These coatings may be applied in single or multiple layers using chemical vapor deposition (CVD) or physical vapor deposition (PVD) techniques, such as sputtering, ion plating and cathodic arc deposition or a combination of these techniques. The rake face of the substrate is roughened to have a surface roughness in the range of 15 microinches Ra to 125 microinches Ra as measured on a Sheffield Proficorder Spectre Unit. Roughening of the rake face of the substrate is believed to promote good mechanical adhesion of the coating onto the rake face of the substrate.
The cutting tools described in the referenced U.S. Pat. No. 5,722,803 have typically been useful for machining of relatively soft metals such as mild steels (e.g., AISI 1008 and AISI 1045 steel), cast iron, aluminum alloys, magnesium alloys, copper and brass alloys. The roughened rake face surface also has improved chip formation in machining these soft materials. However, hard turning of materials having a Rockwell hardness (HRC) of greater than 45 Rc, such as hardened tool and die steels, remains problematic. Cutting tools used to finish machine hardened tool and die steels often have short useful lives due to wear.
An alternative to machining hard materials is grinding. However, grinding requires the use of lubricating fluids which are difficult to dispose of in an environmentally sound manner. Therefore, dry hard turning remains preferred over grinding, despite the costs associated with replacing worn cutting tools, because the expenses associated with the use and disposal of lubricating fluids are avoided.
PCBN cutting tools used in hard turning typically experience wear on the rake and flank surfaces, commonly referred to as crater wear and flank wear, respectively. Crater wear is believed to be a result of chemical interaction between the rake surface of the tool and the metal chip produced during machining. Cutting tools used in hard turning are also subject to flank wear caused by abrasion of the work piece itself against the flank surface of the cutting tool. The wear of cutting tools worsens with cutting time until the work piece loses the desired size and/or finish and the cutting tool must be replaced. Accordingly, a need remains for a cutting tool for hard turning with an extended wear life to increase the cutting time for the tool before replacement is needed, thus improving machining productivity.
This need is met by the coated cutting tool and method of the present invention which are useful for hard turning. The cutting tool of the present invention includes a blank/substrate of polycrystalline cubic boron nitride having a rake surface roughness of no more than about 8 to 10 microinches Ra, having a refractory hard coating containing aluminum applied thereto by one of a CVD or PVD technique or combination thereof. Preferably, the coating includes a titanium aluminum nitride layer (e.g., TiAlN) deposited onto the substrate via a PVD technique in a thickness of at least 2 micrometers, and preferably about 2 to 5 micrometers, and more preferably about 3 micrometers in thickness. The PCBN preferably includes at least 40 vol % CBN in addition to the binder.
The coating may alternatively include a lower layer of aluminum oxide (e.g., Al2O3), deposited with or without underlayers of TiN, TiC, TiCN, TiAlN, TiOCN between it and the substrate, and optionally, with an outer layer, for example, of titanium nitride (TiN). A primary function of the outer layer is to act as a visual wear indicator for the machine operator. This layer is typically lighter colored than the substrate of the tool insert.
The method according to the present invention contemplates the use of a polycrystalline cubic boron nitride cutting tool coated with a titanium aluminum nitride layer applied by a PVD technique for hard turning materials such as hardened steel having a hardness of greater than 45 HRC. The coating may instead include one or more lower layers of aluminum oxide applied via CVD and an upper layer(s) of titanium carbonitride, titanium oxycarbonitride and/or titanium nitride applied via PVD or CVD.